dj No, I can't explain, because I don't know. Do you mean when they were in coalition?
Of the current bunch of MPs who have resigned/been sacked, I know what Zeichner said during the recent election, because Cambridge is a neighbouring constituency.
Cambridge is very strongly Remain and relies heavily on the single market. It has been a LibDem/Labour constituency for ages. Labour won it in 2015 and had a tiny majority, so it was an ultra-marginal.
I thought the LibDems would win it back in 2017, but Zeichner increased his majority. Cambridge ended up with a choice between two very good constituency candidates. The previous LD had been good, but so was Zeichner. Both believe passionately in Remain and Zeichner told voters he would do all he could for a "soft Brexit". I believe it was a matter of conscience for him to keep his word to his constituents and the majority on his Twitter feed support him.
This was the same line pushed by Jeff Smith in Manchester Withington, which is also strongly Remain and had been a LibDem seat, although not so marginal as Cambridge.
I had a number of discussions with my daughter about Labour and Remain. It's clear that she (and I suspect many others) voted for and supported Labour, because she thought they would find some way to soften Brexit. My argument always was that a "soft Brexit" can't happen and has ever been defined - I still don't see how it can. My opinion is that Labour has been less than honest about its stance on Brexit.